Process of producing oxygen



y 1951 F. J. JENNY ETAL PROCESS OF PRODUCING OXYGEN Filed Dec. 5, 1945INVENTORS flan/i; J Jenny N m T T A Edward GKJo/zez'bel Patented May15,1951

PROCESS OF PRODUCING OXYGEN Frank J. Jenny, New York, N. Y., and EdwardG.

Scheibel, Nutley, N. .L, assignors to Hydrocarbon Research, Inc., NewYork, N. Y.

Application December 5, 1945, Serial No. 632,858

11 Claims.

This invention relates to the production of oxygen by the liquefactionand rectification of air, and more particularly to an economical methdof obtaining oxygen in high purity and in high yield without the use ofchemical reagents to effeet the removal of carbon dioxide and moisturepresent in air.

All temperatures herein are in degrees F. and pressures in pounds persquare inch gauge.

Oxygen is commonly produced by partial liquefaction of air andrectification at low temperatures; preferably rectification is conductedin two stages at different pressures. The refrigeration necessery forliquefaction is supplied to the air after it has been compressed andwater-cooled to approximately room temperature, by indirect heatexchange with the efiiuent products of rectification. However, anadditional amount of refrigeration must be supplied to compensate forcold losses resulting from the difference in enthalpy between theincoming air and the outgoing products of rectification and for heatleaks into the system. Methods of supplying this refrigerantionheretofore used, involve compressing at least a portion of incoming airto pressures as high as 3000 pounds and expanding with or without theperformance of work to produce a temperature drop; or compressing allthe incoming air to about 600 pounds and after the air has beenpartially cooled by the products of rectification expanding a portion ofthe air. These methods are wasteful from the standpoint of compressorenergy and require a great deal of equipment in the form of extracompressors, intercoolers and expanders.

For economical operation it is essential to recover the cold content ofthe outgoing products of rectification. This is usually accomplished bypassing these products in heat transfer relationship with the incomingair. In older systems in order to avoid deposition of frost and solidcarbon dioxide in the tubular counter-current heat exchangers throughwhich the air is passed in indirect heat exchange relation with theoutgoing products of rectification, the air is treated in driers andcaustic scrubbers to remove water and carbon dioxide prior to admittanceof the air into the heat exchangers. Even with this treatment theexchangers had to be thawed out regularly to remove the frost (whichterm is used in a generic sense to include both snow and ice) whichcaused stopping up of the apparatus.

More recently it has been suggested to use cold accumulators orregenerators (hereinafter referred to as heat exchangers) of large coldabsorbing capacity through which the warm incoming air and the coldproducts of rectification are alternately passed with periodicallyreversed operation so that streams of warm air are flowed through thesame packing-filled spaces that the cold separated oxygen and nitrogentraversed during the previous step in the process, the high boilingimpurities deposited in these spaces during the passage of airtherethrough being removed by sublimation during the subsequent flow ina reverse direction of the products of rectification. The use of thesereversing heat exchangers in a, process in which the air is compressedto relatively high pressure results in more costly operation from thestandpoint of horsepower requirements because upon every reversal, whichmay take place every three minutes, the volume of compressed air in theheat exchangers is lost and must be again replaced. Moreover, in theoperation of such reversing heat exchangers it is important not to letthe temperature at the exit end of the exchangers drop to a point wherea part of the air becomes liquid because this liquid adheres to thesurface of the exchangers and is Wasted upon reversal of flow. On theother hand the temperature conditions under which the exchangers areoperated should be such as to obtain complete purging of the carbondioxide deposited therein upon reversal of flow which usually requireshaving the air exit end of at least one of the exchangers at a lowtemperature, i. e., at or near the dew point of air.

It is an object of the present invention to pro- 'vide a processforproducing oxygen by the liquefaction and rectification of air in whichmoisture and carbon dioxide are removed from the air without the use ofchemical reagents and which involves the use of reversing heatexchangers through which flow in heat exchange relation the outgoingproducts of rectification and the incoming air, the process beingoperated at relatively low pressures of the order of about 70 pounds toabout 85 pounds gauge so that the loss of compressed air in theexchangers upon each reversal is small and in which the reversing ex-"changers are operated so that moisture and carment and power costs ascompared with existing procedures. Other objects and advantages of thisinvention will be apparent from the following detailed description.

In accordance with this invention a stream of air at about 70 pounds toabout 85 pounds gauge and a temperature of about '70 to about 110 F. ispassed through a path in two or more heat exchange zones in series, eachzone containing at least three fiow paths in heat exchange relation witheach other through two of which pass respectively streams of oxygen andnitrogen products of rectification in heat exchange relation with thepath through which the air is passing. One of the streams flowingbetween the first zone and a second zone is refrigerated, the amount ofcold thus introduced into the process being adequate to compensate forcold losses resulting from the difference in enthalpy between the airintroduced into and the products of rectification withdrawn from theprocess and .for heat leaks into the system. The tempera- At the colderend of the second zone where the oxygen and nitrogen products ofrectification enter and air leaves this zone, there is maintainedbetween these products of rectification and the countercurrent stream ofair a temperature difference in the range of about 5 to about F.,preferably about 6 to about 8 F. This temperature difference is thedifference between the temperature of the air and the weighted averagetemperature of the products of rectification, all temperatures beingtaken at the colder end of the second zone. For the purposes of thisinvention, the weighted average temperature of the products ofrectification is calculated by multiplying the temperature of the oxygenproduct stream by the volume percentage of the stream based on thecombined volume of the products of rectification and adding thereto thecorresponding figure obtained by multiplying the temperature of thenitrogen product stream by its volume percentage. Thus, for ex- U ample,if the rectification system is operated to produce two streams ofsubstantially pure oxygen and pure nitrogen, the weighted averagetemperature of the two streams would be approximately the sum of of theoxygen stream temperature and of the nitrogen stream temperature.Periodically the flow of air and nitrogen through their respective zonesis reversed so that upon reversal the air flows through the pathsthrough which during the preceding step the nitrogen had passed and thenitrogen flows through the paths through which had previously passed theair. The nitrogen removes, by sublimation, the carbon dioxide depositedduring the preceding step in the second zone and the frost depositedduring the precedingstep in the first zone.

Operating in this manner complete purging of carbon dioxide is obtainedupon each reversal of flow. Likewise complete purging of frost is ob-.tained so that the equipment may be operated continuously.

Since the refrigeration necessary to compensate for cold lossesresulting from the difference in enthalpy between the incoming air andthe.

outgoing products of rectification and for heat leaks into the system issupplied at a point in the system where the temperature is relativelyhigh, 1. e., the temperature may be of the order of about l35 F., it canbe supplied economically, i. e., with low power cost. Operating within atemperature range of about F. to about 200 F. carbon tetrafluoride orethylene may be used to supply the necessary refrigeration, or a minorportion of the compressed air, say about 7%, may be expanded for thispurpose.

The apparatus required for the practice of the process is moreeconomical and simpler than that required for priorknown processes inthat the heat exchanger surface is reduced to a minimum and the need forcaustic scrubbers, driers and compressors for high pressure air iseliminated. These economies in equipment and operating costs areobtained at no sacrifice to the oxygen recovery, or to purity ofthedesired oxygen product; recoveries as high as 99% plus ofsubstantially pure oxygen may be obtained when operating in accordancewith the preferred embodiment of the invention. Recoveries as high asabout 93% of the oxygen of the total air introduced into the process maybe obtained in another embodiment in which the refrigeration is derivedby expanding 7% of the air introduced into the system.

In the accompanying drawing Figure 1 illustrates diagrammatically thepreferred layout of apparatus for practicing the process of thisinvention; and

Figure 2 shows afragmentary detail of the layout of apparatus of Figure1, but in which instead of cooling the air stream fiowing from oneexchanger to another, as in Figure 1, the nitrogen stream is cooled bythe refrigerant.

In the drawing, the equipment shown for the practice of the processinvolves a pair of heat exchangers having an ethylene refrigerationsystem for refrigerating the air and the present description will beconfined to the present illustrated embodiment of the invention. It willbe understood, however, that the process may be carried out in otherequipment, for example, each of the two exchangers in series may bereplaced by two or more smaller exchangers placed in series and/0rparallel, if desired, although this is objectionable from the standpointof increasing construction costs, or the number of heat exchange pathsin eachexchanger or zone may be increased over the 3-path constructionshown in the drawing, or other refrigeration systems may be employed inlieu of the ethylene system. Hence the scope of the invention is notconfined to the embodiment herein described.

In the drawing reference character l0 indicates a heat exchanger whichmay be of any well known type. In the embodiment shown on the drawingsitconsists of a single shell in which are provided three flow paths,namely, interior path I I through which flows in one and the samedirection throughout the operation of the exchanger the oxygen productof rectification. Paths I2 and I3 are providedwithin the shell of theexchanger through which periodically flow air and the nitrogen productof rectification in heat exchange relation with each other and with theoxygen. The heat exchanger has in each of the paths suitable fins ofheat-conducting material, e. g., copper, promoting rapid and efficientheat exchange between' the gaseous media flowing therethrough. Astheconstruction of. the heat exchanger per se does not form part of thisinvention and as it may be of any well known type, it is believedfurther description thereof is unnecessary.

The flow of the air and nitrogen through their respective paths isperiodically reversed so that during one step of the process air flowsthrough path 52 and nitrogen through path I3, and upon reversal, duringthe succeeding step air flows through path is and nitrogen through pathl2.

Reversal of flow is accomplished by suitably positioning the compoundreversing valves l 4 and 15 which may be of any well known type. ValveI4 is disposed in the pipe line system consisting of air inlet pipe l6leading into valve I4, and pipe lines I! and i8 leading from the valveto cooling paths I2 and i3, respectively. At the base of the 'heatexchanger Hi lines I9 and 2!] are positioned leading from paths i2 andI3, respectively, to the valve I 5.

A second heat exchanger 2| is provided in the form of a shell havintherein paths 22, 23 and 24 provided with fins to promote heat exchangeas in the case of the exchanger 10. Path 24 is the path through whichthe oxygen product of rectification flows from the rectification systemhereinafter described to a pipe line 25 which communicates with path llof heat exchanger Ill. The base portions of paths 22 and 23 of heatexchanger 2! communicate with pipe lines 26 and 21, respectively, whichare communicably connected with a compound valve 28 which may be of thesame type as valves M and H). At the upper portions paths 22 and 23communicate respectively with lines 29 and 30 which in turn communicatewith a compound reversing valve 3!. which may be of the same type as theother reversing valves.

Reversing exchangers Ill and 2| may be placed in vertical, horizontal orany other desired posiin indirect heat exchange relation with thenitrogen or air to be cooled, the rate of flow and.

temperature of the various media being so controlled that enough cold isintroduced by refrigeration, at this point in the process, to compensatefor cold losses resulting from the difference in enthalpy between theincoming air and the outgoing products of rectif cation and for ,heatleaks into the system. In the preferred embodiment of the invention theair leaving the heat exchanger Ill is refrigerated to cause a drop ofabout 5 to about F. in its temperature; this has been found adequate forthe purposes above stated. Refrigeration of the air is accomplished .bycausing it to flow through pipe line 33 which passes through therefrigerator 32in indirect heat exchange with the refrigerant andcommunicably connects valves l5 and 28. Line 9 is the nitrogen linebetween the two heat exchangers Ill and 2 I.

Instead of refrigerating the nitrogen flowing .from heat exchanger 2! toheat exchanger l0 thereby introducing the cold supplied by the re--Irigerating mediuminto the exchanger l0, Orin- -to cause the flow ofthe refrigerating medium stead of refrigerating the air flowing fromheat exchanger Ill to heat exchanger 2|, the desired amount ofrefrigeration may be introduced into the system by expanding a portionof the air, say about 7%, of the total air introduced into the system.The cold expanded air thus produced may be introduced into the nitrogenstream entering the heat exchanger l0 thereby supplying the necessarycold to compensate for cold losses resulting from the difference inenthalpy between the incoming air and the outgoing products ofrectification and for heat leaks into the system. This latter method hasthe disadvantage that it involves a loss of approximately 7% of theoxygen content of the air introduced into the system. On the other handit has the advantage that it eliminates the necessity for using arefrigeration system for cooling either the nitrogen or air, whichsystem is more cumbersome and expensive in construction and operationthan an expander of the type suitable for expanding a relatively smallamount of air at a relatively low pressure, e. g., '70-85' pounds gauge.

With the arrangement of valves and piping shown flow of nitrogen and airthrough heat exchangers it and 2! may be perodically reversedsay everythree minutes-so that during an initial period of operation air flowsthrough cooling path l2, through line l9, valve 15, refrigeration system32 by way of line 33, valve 28, line 2%, cooling path 22 in heatexchanger 21, pipe line 29, valve 31 and thence to line 34 leadingthrough the non-reversing heat exchanger 35 to the rectification systemhereinafter described. At the same time, nitrogen flows through pipeline 36 leading from the non-reversing heat exchanger 35 into valve 3i,line 30, through path 23 in heat exchanger 2|, through line 21, valve28, line Q, valve l5, pipe line 20, path 13 in heat exchanger l6,leaving this path through pipe line 98 and passing through valve I 4 tothe atmosphere or other suitable disposal point. Upon reversal (as shownby dotted arrows and valve settings), the air flows through valve l4,line l8, cooling path it, pipe line 28, valve 55, refrigeration system32 by way of line 33, valve 28, pipe line 21, and thence through thecooling path 23, leaving this cooling path through pipe line 33 andpassing through valve 3,! and pipe line as into the non-reversingexchanger 35. At the same tune, the nitrogen flows from heat exchanger35 through pipe line 36 into valve 3i thence through pipe line 23, path22 in heat exchanger 2!, pipe line 25, valve 28, line 9, valve I5, linel9 into path 52, thence through line H into valve Hi and thence to theatmosphere or other suitable disposal point.

The rectification system comprises a. two-stage rectification column 3?,the lower section 38 of which is operated at a pressure of about 72pounds gauge and the upper section 39 of which is opgrated at a pressureof from about 4 pounds to about 10 pounds gauge, preferably at about 5pounds gauge. This column as is customary is provided with rectificationplates of the bubblecap or other type. The lower section 38 .cf thecolumn 3"." communicates with a condenser and has a liquid collectingshelf disposed iint'nediately below the condenser so for collectliquiolnitrogen. Pipe line 42 from this shelf 4! to a non-reversing heatexchanger 13 which in turn communicates through a pressure reducingvalve M with the top portion of the Condenser ll! acts a reboiler From:therbase: portionwf. thelowerrsection 38 a:. pipe; line; 46 for;thedizow of'crude oxygen (containing: approximately 40% oxygen) passestoa non-reversing heat exchangerwhich communicates through1pipe; line 48having a pressure reducing; valve 491therein-with the low-pressuresectionr33atan intermediate point indicatedby thereference character 50.A line having a pressure reducing valve- 52 therein, leads fromcondenserdfl'to anitrogen line 53 leading'to-the nonereversing-heat.exchanger 35; A- line, El i-leads fromthe top of low-pressurecolumn 39 to:- the heat-exchanger. 43; they nitrogen flowing throughthis. line passing throughtheheat exchanger 43 then througlrline 55?and" heat exchanger 41' into line 56 which communicates with line 53. Anoxygemlinelfl leads from the lower part of the low-pressure section:390i column S'Z' to path 24 offheatexchangerfli. The heat exchangers 35,4'3-and;4'|:- and the two-stage fractionating columnv3i-may be of anyconventional type.

.Two separate fractionating columns, suitably interconnected, may beused inplace of the twostage column 3? shown. It will be understood thatthe equipment throughout is heat insulated to minimize loss of cold,

For a; desirable operating range, air at a pressure.of 70 pounds to 85pounds gauge and a temperature of 70 to 110 F. is introduced into thecooling path in heat exchanger l0, leaving this path at a temperature of-120 to -l50 F., and substantially the same pressure. The nitrogen andoxygen products of rectification enter the heat exchanger ID at atemperature of -130 to 165 F. and leave at a temperature of 60 to 100 F.The air in passing from heat exchanger li! to heat exchanger 2| isrefrigerated from 5 to F. so that it enters heat exchanger 2| atatemperature of 125 to 160 F. and leaves at a temperature of 260 to 280F.The nitrogen enters heat exchanger 2| at a temperature of 265-to 285 F.and leaves at a temperature of 130 to 165 F. The oxygen enters heatexchanger 2| at a temperature of -288 to 293 F. and leaves at atemperature of -130 to 165 F. The several streams suffer only a smallpressure drop in flowing through the two reversing exchangers in series.

One example of the operation of the process of this invention isdescribed below; it will be understood this example is given forpurposes of exemplification only and the invention is not limitedthereto.

Air under pressure of about 75 pounds gauge and temperature of about 100F. is supplied through line |B, valve Hi and line H to heat exchangerl0, flowing through path |2 in which it is cooled to a temperature of-134 F. The air then flows in indirect heat exchange relation withethylene in the refrigeration system 32 and is cooled thereby to atemperature of -142' F., then passes through the path 22 of the heatexchanger 2| leaving this path at a temperature of 2"73 F. Substantiallyall moisture is removed in the form of frost in path |2 of heatexchanger l0 and all carbon dioxide is removed in solidified form inpath 22. of heat exchanger 2|. The air. then flows through the heatexchanger 35 in heat exchange relation with nitrogen and entershigh-pressure column 38 at a temperature of 275 F. and a pressure of 72-pounds.

Crude oxygen at a temperature ofv Z78 F. and, pressure of 7 2 poundsleaves the base of column.38, flows. through the heat exchanger 41 whereits temperature .-is;reduced to I 286 F. and upon flowthrougnthepressure reducing valve 49 isflashed, entering lowrpressurecolumn 39 ata temperature of 3l0 to 315 F. and a pressure of.5--pounds.- Pure oxygen is withdrawn through line 51: at a-.temperatureof -292 F. and a pres.- sureyofz5-pounds andflows through path 24, itstemperature being increased to 146 F., the oxygen at'thiss temperatureflowing through path ll of. heat exchanger I0, and bein withdrawn fromthis path" at atemperature of F. and, at a pressure; of;1 pound.

Nitrogenat-a temperature of about 286 F. andazpressure of '72 pounds iswithdrawn through line 5| and'passes through valve 52, it tempera.-ture, being reduced; to a temperature of about 315. F. as'aresult'of theexpansion through the pressurereducingvalve 5.2. Nitrogen at atemperature-of 315 F. and a pressure 0L5 pounds is withdrawn throughline 54 and flows through heat exchanger 43 where its temperature israised to about 30.42 F. The .nitrogen flows fromheat exchangen- 432through heat exchanger 41. and mixes with that from-line, 5 the nitrogenstream thusproduced at a temperatureof 294 F. flows throughline:.53.int.o andthrough heat exchanger 35:.where the temperature ofthe nitrogen is raised to Z78? F.. Thev nitrogen at this temperature anda pressureof: aboutv 5 pounds enters path 23 flowing; therethrough. inheat exchange relation with the-oxygenwhich enters at; a temperature of'292'F. and the air which, as above pointed out,. enters the base of?heat. exchanger 2| at 142F. and'leaves at a temperature of -273 F.AtithBCOIdBI. endofthe heat exchanger 2|, the nitrogen andthe oxygenstreams have a weighted average-temperature of nearly -28l F. while atthis point the air i at a temperature of "273." F. A temperaturedifferenceofabout 8 F. is therefore: maintained. It will be noted thatat the point where the air enters and the oxygen'and nitrogenleavetheheat exchanger 2| the difierence in temperature is approximately 4 F.,the air-being ata temperature of 142 F. and the oxygenandnitrogen at 146F.

From the heat exchanger 2| the nitrogen flows through line-21; reversingvalve 28, line 9, reversingvalve I5; line 20 into and through path I3 inheat'exchanger' l0, entering this path at a temperature of: 146'F'. andleaving at a temperature of 90F. and a pressure slightly aboveatmospheric, sayl lipound gauge.

Upon reversal (as shown by dotted arrows and valvesettings), which maytake place-every three minutes; the. air flows through the paths l3 and2.3;:respectively; ofheatexchangers |0 and 2l and nitrogen flows throughpaths l2 and 22, respectively, of. heat: exchangers l0. and 2|. The flowis'otherwise' substantially the same and'the temperature; and:. pressureconditions remain the same. The nitrogen in its flow through path 22 ofheat exchanger 2| removes by sublimation the carbon-dioxide deposited inthispath by the air duringzthe' preceding step. Likewise the nitrogen inits-flow through path I201 heat exchanger Ill removes from this paththefrost deposited therein from;the;air during: the preceding step. Thus inthe; continued-operation upon each. reversal the nitrogeneffects'removal of the carbon dioxide and frostdeposited in; the paththrough which the air had passed during the preceding step of theprocess- The.-exp ressions. reversing the flow of air and nitrogen?andreversal are used. herein in the sense commonly; employed. in.thisart, namely, to

mean the switching of the flow of two streams, for example, the air andthe nitrogen streams, so that upon each reversal the air flows throughthe path through which had previously flowed the nitrogen, and thenitrogen fiows through the path through which had previously flowed theair.

It will be noted that this invention provides a process for producingoxygen of high purity without the use of chemical reagents which processmay be operated continuously for long periods of time without shutdownsfor the purpose of removing solid carbon dioxide or frost, which processis economical to operate particularly in that the refrigeration whichmust be supplied to compensate for cold losses resulting from thedifference in enthalpy between incoming air and the outgoing products ofrectification and for heat leaks into the system is supplied at a pointin the process where the temperatures are relatively high so that it canbe supplied efficiently and economically.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing a stream of air at about 7'0 to about 85pounds gauge through a path in at least two heat exchange zones inseries, each of said zones containing at least three paths in heatexchange relation with each other, passing respectively streams ofoxygen and nitrogen products of rectification through two other paths insaid zones in heat exchange relation with the air passing therethrough,the air thus being cooled to a temperature sufficient to substantiallycompletely remove all moisture therefrom before the air leaves the firstzone, cooling at least one of said streams during its flow between thefirst zone and the second zone, the cold thus introduced into theprocess compensating for cold losses resulting from the difference inenthalpy between the air introduced into and the products ofrectification withdrawn from the process and for heat leaks into thesystem, regulating the fiow of air, nitrogen and oxygen into and fromsaid second zone so that the temperature within said second zone is suchas to effect substantially complete removal of carbon dioxide from theair in its passage through its heat exchange path in said second zoneand the difference between the temperature of the air at the colder endof said second zone and the Weighted average temperature of the nitrogenand oxygen at the colder end of said second zone is within the range offrom about to about 10 F., and periodically reversing the fiow of airand nitrogen through their respective paths in said zones, the air uponreversal flowing through the path through which had previously flowedthe nitrogen and the nitrogen flowing through the path through which hadpreviously flowed the air, whereby upon each reversal the nitrogensubstantially completely removes the carbon dioxide deposited in saidsecond zone during the preceding step of the process.

2. A process as defined in claim 1, in which the stream cooled duringits flow between the first zone and the second zone is the air stream.

3. A process as defined in claim 1, in which the stream cooled in itsflow between the first zone and the second zone is the nitrogen stream.

4. A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing air at about to about pounds gauge and atemperature of 70 to F. through one path in a heat exchange zonecontaining three paths in heat exchange relation with each other,

passing oxygen and nitrogen products of rectification, respectively,through the two other paths in heat exchange relation with the air, theair thus being cooled to a temperature sufficient to substantiallycompletely remove all moisture therefrom, thereafter refrigerating theair leaving said zone, the amount of cold thus introduced into the airbeing adequate to compensate for cold losses resulting from thedifference in enthalpy between the air introduced into and the productsof rectification withdrawn from the process and for heat leaks into thesystem, then further coolin the air by fiowing it through one path in asecond heat exchange zone containing three paths in heat exchangerelation with each other, passing oxygen and nitrogen products ofrectification, respectively, through the two other paths in said secondheat exchange zone in heat exchange relation with the air and therebycooling the air to a temperature sufiicient to remove substantially allcarbon dioxide therefrom, passing the thus cooled air from the saidsecond heat exchange zone to a rectification system in which theaforesaid oxygen and nitrogen products of rectification are formed, andperiodically reversing the fiow or air and nitrogen through theirrespective paths in the said two zones, the air upon reversal flowingthrough the paths in the said two zones through which had previouslyfiowed the nitrogen and the nitrogen flowing through the paths in thesaid two zones through which had previously flowed the air, whereby uponeach reversal the nitrogen substantially completely removes the carbondioxide deposited in the said second zone and the frost deposited in thesaid first zone during the preceding step of the process.

5. A process as defined in claim 4, in which the differential betweenthe temperature of the air and the weighted average temperature of theoxygen and nitrogen at the colder end of the second zone is maintainedwithin the range of about 5 to about 10 F.

6. A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing air at about 70 to about 85 pounds gaugeand a temperature of about 70 to about 110 F. through one path in a heatexchange zone containing three paths in heat exchange relation with eachother, flowing oxygen and nitrogen products of rectificationrespectively through the other two paths in heat exchange relation withthe air, the air thus being cooled to a temperature of about to F., themoisture being thereby removed from the air, refrigerating the airleaving the first zone about 5 to 10 F., then further cooling the air byflowing it through one of the paths in a second zone, flowing oxygen andnitrogen products of rectification respectively through the other twopaths in said second zone in heat exchange relation with the air therebycooling the air to a temperature of about -260 to about 280 F., thedifferential between the temperature of the air and the weighted averagetemperature of the oxygen and nitrogen at the colder end of said assassvsecond zonebeing within the range of about to about F., and periodicallyreversing the flow: of airandnitrogenthrough their respective paths'inthe said two zones, the air upon reversal flowing through the paths inthe two zones-through which had previously flowed the nitrogen and thenitrogen flowing through the paths in the said two zones through :whichhad previously flowed the: air, whereby upon-each reversal the nitrogensubstantially completely removes the carbon dioxide deposited in saidsecond zone and the frost deposited in said first zone'duringthe-preceding step in the'process.

'7. A: processasdefined inclaim 6, in which the air leaving the firstzone is refrigerated'by passage in heat exchange relation with liquidethylene.

. 8. A process .forproducingoxygen by the liquefaction andrectificatiomof air, which comprises passingair under :pressure of about75 pounds gauge and at a temperature of about 110 F. through a path ina'heat-exchangezone containing threepaths in "heat exchange relationwith each other, passing oxygen and nitrogen products of rectificationat a temperature of about 146 F. through the 'other two paths therebycoolingthe: air to a temperature otabout 134 F., passingthe:thus-cooledair in heat exchange relation *witha refrigerating mediumand'thereby cooling: it to'ab'out -142 F., passing i the air at atemperature of about -142 F. through one ,path'in -a second heatexchange zonecontaining three; paths inheatexchange relation witheachothen-passing the oxygen product of rectification 311722.11 initialtemperature of about 292 and the nitrogen product of rectification at aninitial temperature of about *2'78" throughthe other two paths in saidsecond zone in heat 'exchange relation with the air, whereby the air-.is-'cooled to a temperature of about 2'73 F. and the oxygen andnitrogen are heatedtoa temperaturerof about '146 F. upon leaving said.secondzone and substantially all carbon dioxide contained :inthe air isremoved therefrom during its passage'through the secondzone, passing theair from the second zone'tc a rectification system, continu-ouslyflowing the oxygenthrough the same paths in the said two zones,-andperiodically reversing the flow of the air and-nitrogen sothatiuponreversal the air flows through the path through'which the nitrogen hadflowed during the preceding step and the nitrogen flows through the paththrough which the air had'flowed during the preceding step whereby thenitrogen removes carbon dioxide and frost-depositedduring' a precedingstep in the paths through which air had flowed during .the saidpreceding step.

.9. A process as defined in claim -8, in-which the refrigerating mediumis ethylene.

10. A processforpproducing=oxygen by the liquefaction and rectificationof air, which comprises passing astream ofair through a path in at leasttwo heat exchange zones in series, passing a.stream-ofrectificationproduct through another path in-said heat 1 exchange zones in heatexchange relation with the air passing therethrough, the air thus beingcooled to a temperature sufficient to substantially completelly removeall moisture therefrom before the air leaves the first zone, cooling atleast one of said streams during .its flow between the first 12 zone andthesecond zone, the cold thus introduced into the process compensatingfor'cold losses resulting from the difference in enthalpy between theair introduced into and the products of rectification withdrawn from theprocess and for heat leaks into the system, regulating the flow of airand rectification product so that the'temperature within said secondzone is such as to effect substantially complete removal of acondensible constituent from the air in its passage therethrough and thetemperature difference-between the air leaving and the rectificationproduct entering said second zone falls within the range of about'5 toabout 10 F., and

7 periodically reversing the flow of air and rectification productthrough their respective paths in said second zone whereby upon each ofsaid reversals the rectification product substantially completelyremoves the condensible constituent deposited during the preceding stepof the process.

11. A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing a stream of air at about '70 to aboutpounds gauge and a temperature of about 70 to about F. through a path inat least two heat exchange zones in series, passing a stream ofrectification product throughanother .path in-said heat-exchange zonesin heat exchange'relation with the air passing therethrough, the airthus being cooled to a temperature sufficient tosubstantially-completely remove all moisture therefrom before the airleaves the first zone,:cooling-at least one of said streams duringits'flow between the first zone and the second zone, the'cold thusintroduced into the process compensating for cold losses resulting fromthe difference in enthalpy between the air introduced into and theproductsof rectification withdrawn from the process and for heatleaksinto the system, regulating the flow of .air and rectification productinto and .from said .second zone so that the temperature-within said.second zone is such as to effect substantially complete removal ofcarbon dioxide from theair in.its passagethrough said second zone andthe temperature difference between the air leaving and the rectificationproduct entering said second zone falls within the rangeofabout- 6toabout 8 F., and periodically reversing theflow of air andrectification product through their respective paths in said second zonewhereby upon each reversal the rectification product substantiallycompletely removes the carbon dioxide deposited in said second zoneduring the preceding step of the process.

FRANK J. JENNY. EDWARD G. SCI-IEIBEL.

REFERENCES CITED The following references "are of'record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,057,804 Twomey Oct. 20, 19362,406,859 Trumpler Feb. 8, 1949 FOREIGN PATENTS Number Country Date469,943 Great Britain Aug. 3, 1937

1. A PROCESS FOR PRODUCING OXYGEN BY THE LIQUEFACTION AND RECTIFICATIONOF AIR, WHICH COMPRISES PASSING STREAM OF AIR AT ABOUT 70 TO ABOUT 85POUNDS GAUGE THROUGH A PATH IN AT LEAST TWO HEAT EXCHANGE ZONES INSERIES, EACH OF SAID ZONES CONTAINING AT LEAST THREE PATHS IN HEATEXCHANGE RELATION WITH EACH OTHER, PASSING RESPECTIVELY STREAMS OFOXYGEN AND NITROGEN PRODUCTS OF RECTIFICATION THROUGH TWO OTHER PATHS INSAID ZONES IN HEAT EXCHANGE RELATION WITH THE AIR PASSING THERETHROUGH,THE AIR THUS BEING COOLED TO A TEMPERATURE SUFFICIENT TO SUBSTANTIALLYCOMPLETELY REMOVE ALL MOISTURE THEREFROM BEFORE THE AIR LEAVES THE FIRSTZONE, COOLING AT LEAST ONE OF SAID STREAMS DURING ITS FLOW BETWEEN THEFIRST ZONE AND THE SECOND ZONE, THE COLD THUS INTRODUCED INTO THEPROCESS COMPENSATING FOR COLD LOSSES RESULTING FROM THE DIFFERENCE INENTHALPY BETWEEN THE AIR INTRODUCED INTO AND THE PRODUCTS OFRECTIFICATION WITHDRAWN FROM THE PROCESS AND FOR HEAT LEAKS INTO THESYSTEM, REGULATING THE FLOW OF AIR, NITROGEN AND OXYGEN INTO AND FROMSAID SECOND ZONE IS SUCH AS TO EFFECT SUBWITHIN SAID SECOND ZONE IS SUCHAS TO EFFECT SUBSTANTIALLY COMPLETE REMOVAL OF CARBON DIOXIDE FROM THEAIR IN ITS PASSAGE THROUGH ITS HEAT EXCHANGE PATH IN SAID SECOND ZONEAND THE DIFFERENCE BETWEEN THE TEMPERATURE OF THE AIR AT THE COLDER ENDOF SAID SECOND ZONE AND THE WEIGHTED AVERAGE TEMPERATURE OF THE NITROGENAND OXYGEN AT THE COLDER END OF SAID SECOND ZONE IS WITHIN THE RANGE OFFROM ABOUT 5* TO ABOUT 10* F., AND PERIODICALLY REVERSING THE FLOW OFAIR AND NITROGEN THROUGH THEIR RESPECTIVE PATHS IN SAID ZONES, THE AIRUPON REVERSAL FLOWING THROUGH THE PATH THROUGH WHICH HAD PREVIOUSLYFLOWED THE NITROGEN AND THE NITROGEN FLOWING THROUGH THE PATH THROUGHWHICH HAD PREVIOUSLY FLOWED THE AIR, WHEREBY UPON EACH REVERSAL THENITROGEN SUBSTANTIALLY COMPLETELY REMOVES THE CARBON DIOXIDE DEPOSITEDIN SAID SECOND ZONE DURING THE PRECEDING STEP OF THE PROCESS.